Use os.sep instead of hardcoded linux separator

[rjanvier]

What does this PR do?

It makes SPT training work on windows by replacing calls to str.split('/') by str.split(os.sep). It corrects a missing import too.

Before submitting

• [X] Did you make sure title is self-explanatory and the description concisely explains the PR?
• [X] Did you make sure your PR does only one thing, instead of bundling different changes together? Apart the missing imports, yes
• [X] Did you list all the breaking changes introduced by this pull request?
• [ ] Did you test your PR locally with pytest command? Tested on a modified code base of SPT, but not on the upstream repo but it should be harmless
• [ ] Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

moderatly